Conventionally, there are pneumatic tires having a tread surface in which a plurality of main grooves extend straight in the circumferential direction of the tire to thereby define circumferentially extending ribs (land portions), and pneumatic tires having a tread surface in which, in addition to the main grooves, lateral grooves which extend in the widthwise direction of the tire are disposed in predetermined intervals in the circumferential direction of the tire to define circumferentially extending block rows (land portions) by the main grooves and lateral grooves.
In such pneumatic tires, for example, as shown in FIG. 6, there is a pneumatic tire in which the ribs 13 formed between the main grooves 12 have ground contact surfaces 13x defined by a circular arc C1 in the tire meridian cross section, the circular arc C1 having a radius R1 of curvature which is the same as the radius R0 of curvature of a circular arc C0 defining the profile of the tread surface 11.
In pneumatic tires of this type, a ground contact pressure focuses on the outer end 13a of each rib 13 located on the outer side M of a vehicle when the tire is mounted on the vehicle under service conditions which cause a great turning force, such as running of a circuit, and therefore, uneven wear such that the outer end wears locally is apt to occur. In particular, a high ground contact pressure is applied to the outer end 13a of a second land portion 13N when counted from the vehicle outer side M. Therefore, the outer end 13a thereof wears significantly, and there is a problem of deteriorating uneven wear resistance to a large extent.
Also, concentration of a ground contact pressure on each outer end 13a causes the inner end 13b of each of the land portions 13 to be susceptible to lifting and sliding. Especially, the inner end 13b of the second land portion 13N is greatly lifted and slid, resulting in creation of a problem of lowering a turning force.